Method and apparatus for measuring subsurface electrical impedance utilizing first and second successively transmitted signals at different frequencies



Aug. 1s, 1970 fiw/ T. R. MADDN ETAL 3,525,037

METHOD AD APPARATUS FOR MESURING SUBSURFACE ELECTRICAL IIIPEDANCEUTILIZING FIRST AND SECOND SUCCESSIVELY Filed Nov. 14. 1967 lTRANSIITTED SIGNALS AT DIFFERENT FREQUENCIES 4 Sheets-@heet l l SEARCH'Hom Aug. 18, 1970 T. R. MADDEN ETAL 3,525,037

METHOD AND. APPARATUS FOR MEASURING SUBSURFACE ELECTRICAL IMPEDANCEUTILIZING FIRST AND SECOND SUCCESSIVELY TRANSMITTED SIGNALS AT DIFFERENTFREQUENCIES 14. 1967 4 Sheets-Sheet 2 Filed Nov.

Aug. 18, 1970 T. R. MADDEN ETAL 3,525,037

METHOD AND APPARATUS FOR MEASURING SUBSURFACE ELECTRICAL IMPEDANCEUTILIZING FIRST AND SECOND SUCCESSI'IELY TRANSMITTED SIGNALS ATDIFFERENT FREQUENCIES med Nov. 14. 19m 4 mts-snm 5 Aug. 18, 1970 T, R,MADDEN ETAL 3,525,037

METHD AND APPARATUS FOR MEASUBING SUBSURFACE ELECTRICAL IMPEDANCEUTILIZING FIRST AND SECOND SUCCESSIVELY TRANSMITTED SIGNALS AT DIFFERENTFREQUENCIES Filed Nov. 14. 1,967 4.sheets-fsm@1..1`

@Lf-f 3,525,037 METHOD AND APPARATUS FOR MEASURING SUBSURFACE ELECTRICALIMPEDANCE UTI- LIZING FIRST AND SECOND SUCCESSIVELY 'TRANSMITTED SIGNALSAT DIFFERENT FREQUENCIES Theodore R. Madden, Weston, George H. Hopkins,Ir.,

Boston, and Michael Chessman, Watertown, Mass., assignors, by mesneassignments, to Ampex Corporation, Redwood City, Calif., a corporationof California Filed Nov. 14, 1967, Ser. No. 682,803

Int. Cl. Gtllv 3/12 U.S. Cl. 324-7 2 Claims ABSTRACT F THE DISCLOSURE Areceiver for measuring the subsurface electrical irnpedance of the earthas indicated by the frequency dependent attenuation of low frequencytransmitted signals. The receiver has a gain control for establishing areference gain level at one frequency and an indicator for the signalamplitude deviation at a second frequency. A synchronous detectorcontrolled by the output of a local oscillator synchronized with thereceived signal permits operation under poor signal-to-noise conditions.

Certain aspects of geologic investigation make use of the phenomenon ofinduced polarization in which electrical currents are applied to theground, inducing an electrical surface polarization in certain metallicand other deposits. The polarization currents are measured, providingdata which aids in determining the presence and extent of the deposits.One method of measuring involves the transmission through the ground ofsignal at two low frequencies, as 0.3 and 3 cycles per second. Thesignals are received at a point spaced some distance from thetransmitter and the ampltiudes of the received signals are compared. Thesignal ratio is a measure of the change of subsurface electricalimpedance with frequency and an indication of certain types of mineraldeposits.

This invention is concerned with a receiver for electromagnetic waves,useful in such as investigation. Brieliy, the receiver includes abandpass amplifier, a detector and a display. One feature of theinvention is that the receiver includes a gain control which isadjustable during reception of a signal of one frequency to establish areference gain level, and a signal amplitude indicator which is operableduring reception of a signal at a second frequency to indicate, directlyin percentage, the difference in the amplitudes of the two receivedsignals,

Another feature of the invention is that the receiver has a plurality ofAC-coupled serially connected amplifying and filtering stages which arearranged in the sequence of low pass, bandpass and high pass, to obtaina maximum signal-to-noise ratio. Furthermore, the amplifier gains are sorelated that the amplifiers saturate in sequence from output to input,with an increase in signal strength. The gain of the first amplifier'contributes to signal gain with minimum noise introduction. Thereceiver has a high impedance, AC-coupled input, 'which renders itsoperation relatively unaffected by variation in electrode-ground contactresistance and entirely unaffected by electrodeground contact potential.

' As intimated in the preceding paragraph, noise is a major problem inthe measurement of induced polarization. Several kinds of noise arefrequently encountered. They include earth currents resulting fromtluctations of the earths magnetic iieldwhich at higher frequenciesmerge with intererence from thunderstorms. Manmade interference may becaused by electrical equipment in United States Patent O icve nearbymines and, in urban areas, `from transients on power lines and the like.

Still another feature of the invention is the provision of a full wavephase locked synchronous detector circuit which greatly reduces noiseinterference. The receiver includes a local oscillator locked in phasewith the received signal which in turn generates signals which controlthe operation of the synchronous detector.

Further features and advantages of the invention will readily beapparent from the following specification and from the drawings, inwhich:

FIG. 1 is a diagrammatic illustration of a transmitter and receiver usedin induced polarization impedance measurements;

FIG. 2 is a block diagram of the receiver;

FIG. 3 is a schematic diagram of the bandpass amplifier, detector andindicator circuitry; and

FIGS. 4 and 5 together are a schematic diagram of the local oscillatorand phase lock circuitry which generates the detector control signal.

This application does not include a theoretical discussion of thephysical characteristics which give rise to the induced polarizationphenomenon, or of the interpreta-I tion of survey results. Furtherinformation concerning these aspects of induced polarization surveyingcan be found in:

(1) Marshall, D. J., and Madden, T. D., Induced polarization: a study ofits causes, 1959, Geophysics, 24, 790-816;

(2) Keller, G. V., and Frischknecht, F. C., Electrical methods ingeophysical prospecting (International series of monographs inelectromagnetic waves, volume 10) 1966, Pergamon Press Inc., 517 pps.

During the course of the following specification, and in a table at theend thereof, are descriptions of portions of the circuitry of thereceiver including information concerning component types and values. Itis to be understood that this detailed information is given for thepurpose of making a complete disclosure of a preferred embodiment of theinvention. The various components are not to be considered criticalunless otherwise indicated.

In FIG. 1 of the drawings, a square wave transmitter 10 is shown havingits output connected at 11 lwith the surface of the ground 12. Thetransmitted electromagnetic waves illustrated diagrammatically at 13pass through the ground and induce polarization in certain subsurfaceformations 14. The attenuated signal 15 from transmitter 10, is coupledthrough electrodes 16 to receiver 17. Interfering signals 18 (as fromoperations in a mine or currents induced in the earth from othersources) reach the receiver 17 from below. Interference signals 19 maybe caused by activities above the ground, as transients on a power line.

The receiver is illustrated in block form in FIG. 2 where the electrodes16 are connected with an amplifierilter 25. The output of theamplifier-filter is connected with the detector 26, the output of whichis in turn connected with an output signal indicator 27. Theanrplitier-tlterlZS has three stages, 29, 30 and 31, each of whichincludes an amplifying element and frequency selective circuitry forminga portion of the filter. A gain control 32 acts on each of the stages,as indicated by the broken lines, and controls the overall gain of thereceiver. Each of the amplifying stages 29, 30 and 31 includes provisionfor frequency switching, as will appear, so that the proper pass bandcharacteristics may be established for each signal frequency used.

Detector 26 includes a full wave rectifier 35 which is utilized when thesignal-to-noise ratio is suicient to provide a discernible signal. Underpoor noise conditions, a synchronous detector 36 is utilized and in thisoperating condition the rectier 35 serves as a signal inverter toprovide an input for the synchronous detector. The received signal isalso coupled to a quadrature detector circuit 37 which in turn controlsthe operation of an oscillator 38, the output of which is connected to asignal generator 39 having an in-phase output controlling thesynchronous detector 36 and a quadrature output connected withquadrature detector 37.

The detected signal, either from rectifier 35 or from synchornousdetector 36, is connected with filter circuit 40 of indicator 27. Theoutput of the filter, which is essentially a DC potential, with anamplitude directly related to the amplitude of the received signal, isconnected with meters 41 and 42. Although the meters respond to the samesignal, they have different sensitivities and are utilized in differentmanners to indicate different information concerning the receivedsignals.

In operation a sequence of readings is taken in the following manner. Asignalv at a first frequency is generated by transmitter and is utilizedfor calibration of the receiver. Gain control 32 is adjusted toestablish a reference gain level for the receiver, as indicated by meter42. Transmission of the first ysignal is then terminated. A secondsignal at a different frequency, generally lower than the first, is thentransmitted. The frequency characteristics of amplifier 25 are changedto correspond with the frequency of the second signal but the gainsetting is left unchanged. The difference in attenuation of the twosignals is indicated directly in percentage on meter 41. Following thismeasurement, the transmitter and receiver are reset for the frequency ofthe first signal and the receiver output checked to make sure thereference conditions are still correct. If they are not, the procedureis repeated or a suitable adjustment is made in the impedance deviationdata.

The receiver to be described has provision for four differentfrequencies selected by a 9-section switch 45. All of the sections ofthe switch will not be described in detail. However,`for the sake ofconvenience, each of the sections is designated on the drawing by thenumeral 45 followed by a lower case letter a-i, and each position of theswitch by a number 1 through 4, for example, 45b3. The componentsswitched in the circuit are identified by the switch contact designationand the values are listed in a table at the end of the specification,for the following frequencies:

Switch position: Frequency (cps.)

A coarse gain control for the amplifier is provided by a three sectionll-position switch 46 identified in a manner similar to that for thefrequency switch 45. The gain control switch selects signals fromresistance voltage dividers, the components of which are identified bythe contact numbers. The setting of gain switch 46 indicates the minimumamplitude of the input signal necessary to achieve a selected amplitudeof output signal from indicator 27. In a specific circuit, theserelationships are as follows:

Input signal amplitude Bach stage of the amplifier-filter 25 has as itsactive element an operational amplifier, type PP25a, manufactured byPhilbrick Researches, Inc., the frequency and gain characteristics ofwhich are determined by the relative values of the resistors andcapacitors utilized in the input, output and feedback networks for eachamplifier. All of the components important to operation are identifiedon the drawing and their values are givenin a table at the end of thespecification even though unany are not specifically discussed herein.

The received signal from electrodes 16 is coupled through seriescapacitor and resistors 51, 52 to the input of amplifying element 53.Oppositely connected diodes 54 between the amplifier input and ground 55protect the amplifier from input overvoltages. The output of theamplifier is developed across a voltage divider string forming a part ofgain control switch 46a and is coupled through gain controlpotentiometers 45e (one for each frequency) to a fine gain controlpotentiometer 60. In the feedback network for amplifier 53, a seriescapacitor 45b shunts resistor 61. A capacitor 45a is connected from theinput circuit to ground. Gain switch section 46a selects a portion ofthe output signal to be coupled into the feedback network. The feedbackis degenerative in character so that as a greater portion of the outputis fed back, a stronger signal at the input is required to establish thereference level of output.

A capacitor 62 is connected in shunt with the portion of the voltagedivider string 46a which is included in the feedback network and servesto suppress undesired responses to the system at high frequencies abovethe upper cutoff point of the filters.

The movable tap of fine gain control potentiometer connects a portion ofthe output signal from the first amplifier stage with the input of thebandpass amplifier filter 65.

The output of amplifier 65 is similarly developed across a seriesresistive voltage divider 46b connected with ground. The feedback signalis coupled from the divider through the parallel combination of resistor68 and capacitor 45e to the amplifier. Capacitor 69 shunts the portionof resistor string 46 across which the feedback signal is developed tosuppress the effect of the resistor string 46 on the high frequencyresponses of the filter.

In the third stage or high pass amplifier filter, amplifying element 74has its feedback network connected through resistor 75 directly to theamplifier input and through capacitor 45h across resistor 76 to theinput network between capacitor 45f and capacitor 45g. Capacitor 77shunts the portion of voltage divider 46c in the feedback network,suppressing high frequency responses as in the preceding sections.

The overall frequency response of the receiver is the same regardless ofthe order of the filters. However, the signal-to-noise ratio of theamplifier circuit is greatly improved by arranging the filters in theorder of low pass, bandpass and high pass. By having the low pass filterfirst, there is a maximum protection of the amplifier from highfrequency noise such as that at power line frequencies (commonly 50 or60 cycles per second). The capacitor 50 provides AC coupling into thereceiver, rendering it unaffected by DC contact potential on theelectrodes. The low pass filter is followed by a bandpass filter whichserves to reduce even further the relative amplitude of high frequencynoise, in addition to providing a cutoff at the lower end. The thirdstage has a high pass response and completes the attenuation offrequencies below that being received. As the signal amplitudeincreases, each of the amplifying elements 53, 65, 74 is subject tosaturation. The coarse gain control settings for the amplifiers are sorelated that the amplifying elements will saturate in reverse order,amplifier element 74 saturating first, 65 second and 53 last. i

The output of amplifier-filter 25 is essentially a sine wave at thefundamental frequency of the square wave which was received. All higherharmonics are so greatly attenuated that their effect can be neglected.

Detector 26 has two modes of operation, normal (N) and phaselock (e)selected by two-position, three-section switch )80. In the normaldetection mode operational amplifier 81 and diodes 82, 83 function a's afull wave rectifieij, the full wave output bengcoupled through switchsection- 80e to indicator circuit l27. In the phaselock mode ofoperation, a pair of field effect transistors 93, 94, 2N3821,' operateas choppers controlled by a signal in phase with the received signal,forming a full wave synchronous detector. Further details of generationof the synchronizing signal will be given below. In the phaselock mode,operational amplifier 81 operates as an inverter to provide one phase ofthe input signal to synchronous detector 93 while the incoming signal isconnected directly with detector 94.

In either normal or phaselock operation, the full wave rectified signalis present at lswitch section 80e and is coupled to the input of filteramplifier 105, the output of which is connected with meters 41 and 42.The feedback network for operational amplifier 105 is completed througha 6-position, 3-section switch 106 which establishes the time constantfor the amplifier. In the interest of making rapid measurement, it isdesirable to keep the time constant as low as possible. However, wherethe ,received signal is varying in amplitude because of the Switchposition: Settling time in seconds A reference bias circuit is connectedwith the second input of amplifier 105. The bias source 107 includes aseries current regulator 108 utilizing a field effect transistor and ashunt connected Zener voltage regulator 109i. Potentiometer 110 isadjusted toapply a reference voltage of -0.8 of a volt to the amplifierinput. The receiver gain is balanced when the DC component of therectified received signal is 0.8 volt, `so that the output of filteramplifier 105 is zero. When the detected signal differs from '-0.8 volt,the polarity and amplitude of the amplifier output indicate thisdifference. j

Volt'meters 41 and 42 are connected with the output of amplifier 105.The percent deviation meter 41 has three different ranges, provided byseries multiplier resistors 115, 116, 117. Both meters are protectedagainst voltage surges by shunt connected diodes and capacitors. Meter42 is calibrated to read from zero to 200 with the center null positiondesignated 100 and representing a balaned condition of amplifier 105. Insetting up the receiver, during reception of the first or referencesignal, the gain of the amplifier is initially adjusted until meter 42reads 100. In this condition, the direct current component of thedetected received signal balances with the reference signal from biassource' 107. In adjusting the receiver gain, gain controlpotentiometer`60 is kept at its maximum position while coarse v.gainadjustment 46 is set to produce an output slightly greater than the 100indication on meter 42. The gainis then reduced to the desired level byadjustment of potentiometer 60. Potentiometer 60 is blocked to preventits being set below onequarter of its maximum output, or some of thecalculations which may be made using the gain settings would be subjectto inaccuracy, Final adjustment of the gain control may be madeutilizing the more sensitive meter 41 when set on its most sensitiverange.

After the gain of the receiver has been adjusted, as outlined above, asignal is transmitted at the second frequency and the receiver bandswitch 45 changed to the new frequency without disturbing the gainsetting. The percent deviation of signal amplitude is indicated'directlyby meter 41.

Under conditions of high noise, an improved operation can be achieved byutilizing the phaselock synchronous detector circuit. For synchronousdetection, it is necessary to have a reference signal which has the samephase and frequency as the-transmitted signal. In accordance with theillustrated embodiment of the invention, the reference signal is`generated in the receiver and is synchronized with the received signal.Briefly, relaxation oscillator 538 is synchronized with the incomingsignal and generates a series of pulses at a frequency four times thatof the received signal. The output of the oscillator is coupled througha shaping circuit to a diode gating network '121, which generates aseries of trigger pulses for a pair of bistable multivibrators 122, 123.The output of multivibrator 122 (illustrated at the right of FIG. 5) istwo square waves, one in phase with the received signal (qt) and other180 out of phase Bistable multivibrator 123 operates in quadraturerelation with the received wave and has two outputs q and 180 apart. Thein-phase signal controls the operation of synchronous detector 36 whilethe quadrature signals provide a reference for the quadrature detector37.

Considering the operation ofthe circuit in more detail, the output ofhigh pass amplifier 31 (FIG. 3) is conne'cted with a pair of quadraturedetectors 130, 131, field effect transistors 2N3821, with the q andsignals applied to their bases. When oscillator 38 is properlysynchronized with the received signal, the output of detectors 130, 131is zero. If the oscillator is not properly Synchronized, there will -besignals in the detector outputs which are filtered and connected withtheinput operational amplifier 132. The amplifier output is a DC controlvsignal having polarity and amplitude which represent the direction andamount of oscillator error. The control potential is connected throughfrequency selector switch 45t' with oscillator 38, which utilizesunijunction transistor 133 connected in a relaxation oscillator circuit.The basic oscillator frequency is controlled -by the values of theresistors and capacitors in the-circuit and Ais modified by the controlvoltage. The general operation is known and will not be described indetail. A phase error meter 134 indicates the amplitude of the controlpotential. This serves to warn the operator not to make a'rneasurementwhen the oscillator is improperly adjusted. When the system is badly outof synchronization, a bias potential can b e applied to oscilator 38 byoperation of switch 135 to aid in reaching a synchronous condition.

The pulse output of relaxation oscillator 38 (FIG. 4) is differentiatedand connected with a monostable multivibrator 137 (FIG. 5). The outputof the monostable multivibrator is a clock pulse 138 of accuratelycontrolled amplitude and width. Gate circuit 121 is made up of aplurality of interconnected diodes which. establish trigger pulses forVthe bistable multivibrator 122, 123, in accordance with the clocksignals representing 4the output of synchronized oscillator 38. Thetrigger signal for in-phase multivibrator 22 may be represented by thelogical equation The trigger signal for quadrature multivibrator 123 isrepresented by the logical equation Tq=c`+c`i q u where Tc is thein-phase trigger Tq is the quadrature trigger 7 C is the clock pulse isthe in-phase output signal q is the quadrature output signal..

The output of in-phase multivibrator 122 is connected through bufferamplifiers 140 with the control elements of choppers 93, 94 in thesynchronous detector circuit (FIG. 3).l For purposes of the presentinvention, it may be considered that the synchronous detector translatesthe'signalifrequency to a direct current. All of the noise which pasbesthrough amplifier filter 25 is also translated so that it is symmetricalabout the zero frequency axis. The long time constant filter associatedwith the meter circuit serves as a very narrow band, high Q lter,centered 'on the signal frequency and rejects noise accompanying thesignal in inverse proportion to the lter Q. Use of the quadraturedetector permits operation of the equipment with a signal to noise ratioof the order of one at the output of the bandpass filters. This greatlyextends the utility of the equipment, permitting its operation at timesand in areas where noise has prevented operation in the past.

TABLE OF REPRESENTATIVE COMPONENT VALUES Reference numeral: Description45a 1 ,uf 1.0 2 uf-- 0.33

45`b 1 pf-- 0.056 2 uf..- 0.018

45d 1 ;Lf-- 1.0 2 pf 0.33

45e 1 uf-.. 0.47 2 nf 0.1568

45) 1 /.tf 10.0 2 pf 3.33

4 pf-- 0.33 45g 1 ,.f-- 10.0 -2 pf 3.33

4 f 0.33 45h 1 f 10.0 z ,.f-- 3.33

4 lrf..- .33

451' 1, 10K pot K0 5.6 2, 50K p01 K0 6.8

3, 100K pot Ko 56 4, 500K pot K 68 Reference numeral: Description 50;Lf-- 2 51 meg0-- 10 52 meg0 1.82

61 meg0 10 62 ,uf.... .047 46c 64 --meg0 2 V 68 meg0 2.8

meg0-- 1.05

10611 1 ,uf 0.27 2 nf-.. 0.82

106e 1 at 0.012 2 uf 0.039

108 T1558 109 IN4565A 110 K0 5 K0 15.8 116 K0 71.5 117 K0 5.76 1222-2N1306 123 2-.2N1306 133` 2N1671C 137 2-2N1306 meg0 20 146 meg0 2 147meg0 2 While we have shown and described certain embodiments of ourinvention, it is to be understood that it is capable of manymodifications. Changes, therefore, in the arrangement and constructionmay be made without departing from the spirit and scope of the inventionas disclosed in the appended claims.

We claim:

1. A receiver for measurement of subsurface electrical impedance,utilizing transmitted signals at different frequencies, comprising:

means for receiving said transmitted signals;

an amplifier connected with said receiving means, having selectable bandpass characteristics for the transmitted signal frequencies;

means for seleeing one of said band pass characteristics for saidamplifier;

an amplifying element withy an input and an output in said amplifier;

a resistive-capacitive feedback network connected from the output to theinput of said amplifying element;

a resistive voltage divider connected in shunt with the output of saidamplifying element;

gain adjusting means for connecting a selected portion of said voltagedivider` in said feedback network, operable during reception of a signalata first of said frequencies to establish a reference gain level;

a capacitor connected in parallel with the portion of said voltagedivider which is connected in the feed-y back network; and

an output signal indicator connected with said amplifier, operableduring reception of a signal ata second of said frequencies to indicatethe deviation of signal amplitude at the second frequency with respectto the signal amplitude at the first frequency.

2. The method of measuring subsurface electrical impedance utilizingfirst and second successively transmitted signals at first and secondfrequencies, comprising:

tuning a receiver to the first frequency to receive said first signal;

providing a source of bias potential;

adjusting the gain of an amplifier in said receiver to match thereceiver output from the irst of said transmitted signals to a nullreference level with respect to said source of bias potential;

tuning the receiver to the second frequency to receive said secondsignal while maintaining the ampliiier gain at the adjusted level;

deriving an output from said receiver, the amplitude of which is ameasure of the deviation of the ampli tude of the received signal at thesecond of said frequencies from the amplitude of the received signal atthe first of said frequencies; and

pedance.

providing an indication of the amplitude of said output as a measure ofthe subsurface electrical im- References Cited UNITED STATES PATENTSFOREIGN PATENTS Canada.

GERARD R. STRECKER, Primary Examiner U.S. C1. X.R.

